Cavity QED:
Research Update

 Scientific Progress and Accomplishments:

Over the past year, a completely new laboratory has come online for investigations of quantum state synthesis and quantum logic within the setting of cavity quantum electrodynamics (QED). In addition, there has been significant experimental progress related to the QUIC goals in other laboratories in the Caltech Quantum Optics Group, as for example in the attainment of record finesse for an optical resonator. Finally, several theoretical analyses have been carried out which open new avenues for the realization of quantum networks for distributed quantum computing and for quantum communication.

1. Cold atoms and cavity QED – The principal paradigm that we are pursuing for the implementation of quantum logic is that of the interaction of single atoms and photons within the setting of cavity QED. On the one hand, single photons can serve as the carriers of quantum information (qubits), with interactions between photons that are required for a quantum gate being accomplished via an atom in an optical cavity. Such an arrangement was the basis for our previous demonstration of a quantum phase gate. On the other hand, internal states of atoms can serve as the qubits, with now interactions between atoms localized in a cavity mediated via single photons in the intracavity field. Such a configuration can form the basis of a general purpose quantum computer, as P. Zoller’s group theoretically demonstrated in 1995.

From an experimental perspective, there are major technical hurdles associated with either of these schemes. The most significant of these is the absence of technical capability to date for the localization of single atoms within a high Q cavity in a setting for which strong coupling is achieved. Our prior work has involved atomic beams for which transit times are a few hundred nanoseconds, which was sufficient for an initial demonstration since the “clocking” times for the logic operations would be less than 10 nsec. However, further progress toward the implementation of logic networks requires the isolation of atoms (or ions) inside an optical cavity.

With this in mind, at the outset of the QUIC grant we undertook the construction of a completely new laboratory with the intent of trapping atoms inside a high Q optical cavity. The first stage of this work has come to fruition in the past year with the demonstration of real-time cavity QED with one and the same atom. We have combined the techniques of laser cooling and trapping with cavity QED to achieve a system with saturation photon number of only 2x10-4 intracavity photons with atomic transit time of 100 ?s. Some idea of the magnitude that this advance represents is to note that all prior work in cavity QED with strong coupling (including our own) had after more than a decade attained a product of coherent coupling g and interaction time T of gT ~ ?, whereas our recent experiments achieve gT ~10,000?.

This advance now places us in a position to trap and localize atoms in the cavity. Based upon our ability to monitor the atomic motion near the quantum limits, we are implementing a strategy based upon quantum feedback (in collaboration with Professor D.F. Wall’s group in Auckland and Professor S. Lloyd’s group at MIT). Initial measurements demonstrate that by employing quantum sensing and actuation, a falling atom can be decelerated leading to increased dwell times by a factor of 3-4 for an atom in the cavity. We are currently working to learn (both theoretically and operationally in the lab) the rules and regulations for such a quantum servo in order to trap and cool an atom indefinitely in the cavity.

2. Other experimental progress relates to the investigation of new types of optical resonators for the implementation of quantum logic in cavity QED, which we have previously shown in theoretical work to be very promising.  After some years of struggling, the past year has seen a major advance in our work with the whispering gallery modes of quartz microspheres. We have now attained cavity quality factor Q = 8 x 109 for wavelengths in the near infrared, which corresponds to the highest finesse yet recorded for an optical resonator (finesse F =  2.2 x 106). Perhaps more importantly, we now have quantitative experimental evidence as to the factors that limit the Q (as corroborated by a theoretical model that we have developed); we are now working to reduce these losses. In addition to this work on the cavity Q, we have also obtained the first experimental measurements for cavity interactions of atoms with the external evanescent fields of the whispering gallery modes.

3. On a theoretical front, we have proposed and analyzed a high efficiency scheme for generating a deterministic bit stream of single photon pulses. This work attempts to solve an outstanding difficulty for quantum logic with single photons, namely that to date there has yet to be a laboratory source for single-photon pulses. The basic arrangement involves an atom in a cavity, with the setup described in Section 1 above having actually been analyzed.

Somewhat more generally, together with P. Zoller and I. Cirac, we have developed a scheme for the realization of quantum networks utilizing photons and atoms in cavities. Multiple atom-cavity systems located at spatially separated “nodes'' would be interconnected via optical fibers to create a quantum network (QN) whose unique and powerful properties have been anticipated by recent advances in quantum information theory. Indeed, a complete set of elementary network operations has been proposed and analyzed including local processing of quantum information, transmission of quantum states from one node to another, and the distribution of quantum entanglements. It should be emphasized that these protocols are fully realistic and well within reach of our emerging technical capabilities. With respect to quantum computation, we have proposed a distributed paradigm for ultrascale quantum computing that has the potential to overcome size-scaling and error-correlation problems through the use of a multiple processor architecture.

Other theoretical work in our group relates to the problem of the theoretical understanding of the marriage of the external atomic center-of-mass degrees of freedom with the internal quantized cavity field and atomic dipole. Such work becomes essential as we move toward the actual laboratory localization of atoms in cavities.

Plans for the coming year:
1. The principal experimental thrust will be to trap, cool, and hence localize an atom (or a small collection of a few atoms) in a high-finesse cavity. Two strategies will be pursued, with the first being a continuation of the work described in the preceding section, namely the use of quantum servo control of the atomic motion. The second involves auxiliary (classical) fields injected into the cavity for cooling via polarization gratings and for trapping via a dipole-force trap tuned far from the resonant cavity QED interaction.

2. Given an atom trapped in a cavity, we plan to implement protocols for quantum state synthesis (principally directed to the generation of a bit stream of single photon pulses and to entangled states of the field).

3. A theory for quantum teleportation of continuous quantum variables has been developed. This work will be extended to a more complete time-frequency analysis. As well, an experimental program to accomplish the teleportation of continuous quantum variables is being undertaken.
 
 

List of Participating Scientific Personnel:

Dr. H. Jeff Kimble, Ph.D., Principal Investigator
Dr. Michael Chapman, Postdoctoral Scholar
Dr. Jun Ye, Postdoctoral Scholar
Christina Hood, Graduate Student, Caltech
Theresa Lynn, Graduate Student, Caltech
Quentin Turchette, Graduate Student, Caltech
David Vernooy, Graduate Student, Caltech
Hideo Mabuchi, Graduate Student, Caltech
Visitors: Dr. Samuel Braunstein, Dr. Christoph Naegerl, Dr. Eugene Polzik, Jens Lykke Sorensen, Dr. Crispin Gardiner, Prof. Peter Zoller, Dr. Juergen Mlynek, Mr. Michael Romalis, Dr. Jun Ye, Dr. Tilman Pfau, Dr. Hoi-Kwang Lo, Ike Chuang, Prof. Holevo, Chi-Sheng Niu

 List of Manuscripts/Publications:

1. “Well-Dressed States for Wavepacket Dynamics in Cavity QED,” D.W. Vernooy and H.J. Kimble, Phys. Rev. A56 (Nov. 1997), 4287.
2. “Quantum State Transfer and Entanglement Distribution Among Distant Nodes in a Quantum Network,” J.I. Cirac, P. Zoller, H.J. Kimble and H. Mabuchi, Physical Rev. Letters, Vol. 78, No. 16 (April 1997), 3221.
3. “Deterministic Generation of a Bit-Stream of Single-Photon Pulses,” C.K. Law and H.J. Kimble, J. Mod. Optics 44, 2067 (1997).
4. “Nonclassical Light Twenty Years Later - An Assessment of the Voyage into Hilbert Space,” H.J. Kimble, to be published in Philosophical Transactions A: The Royal Society (1997).
5. “Teleportation of Continuous Quantum Variables,” S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. (Submitted Sept., l997).
6. “Strong Interactions of Single Atoms and Photons in Cavity QED,” H.J. Kimble, Submitted for publication to Physica Scripta; The Royal Swedish Academy of Sciences (1997).
7. “High Q Measurements for Fused Silica Microspheres in the NIR,” D.W. Vernooy, V.S. Ilchenko, H. Mabuchi, E.W. Streed and H.J. Kimble, Opt. Lett. (to be published).
 8. “Cold Atoms in Cavity QED: Quantum Measurements and Quantum Traps,” M.S. Chapman, C.J. Hood, T.W. Lynn, H. Mabuchi, Q.A. Turchette and H.J. Kimble, Proceedings of 13th International Conference on Laser Spectroscopy (June 3, 1997).
9. “Quantum Structure and Dynamics for Atom Galleries,” D.W. Vernooy and H.J. Kimble, Phys. Rev. A55, 1239 (February l997).